2023 March Board Book

Pressman et al.

10.3389/fsufs.2022.1072805

factors for California dairy cattle remained constant from 2012 to 2020 (CARB, 2022a). In contrast, U.S.-wide emissions factors increased by 8.7% from 2010 to 2020 (EPA, 2022). The relative stability of California dairy enteric CH 4 emissions factors may reflect interplay between increasing milk production and improvements in feed efficiency. Increased per capita milk production could associated with greater feed intake and thus increasing enteric CH 4 emissions factors, as both CARB and EPA develop enteric CH 4 emissions factors CH 4 conversion rate, which is the fraction of gross energy (GE) in feed converted to CH 4 , and GE intake increases with increasing net energy for lactation (NE L ), which itself increases with increasing milk production (IPCC, 2006; CARB, 2022a; EPA, 2022). However, a life cycle analysis comparing California dairy environmental footprints in 1964 and 2014 found that in 1964, the feed conversion rate was 1.93 kg feed per kg energy-corrected milk (ECM), while in 2014, the feed conversion ratio was 0.79– 0.81 kg of feed/kg of ECM, suggesting cattle today utilize feed more efficiently than those 50 years ago. In 1964, each cow emitted 0.98 kg of CO 2 equivalents of enteric methane per kg ECM compared with 0.43–0.45 kg of CO 2 equivalents of enteric methane per kg ECM in 2014 (Naranjo et al., 2020). Average ECM production in 1964 was 15.73 kg/day, while it was 39.8 kg/day in 2014, making enteric methane emissions factors 15.4 kg CO 2 equivalents per day in 1964 and 17.11–17.9 kg CO 2 equivalents per day in 2014. Previous authors have predicted future inventories of livestock methane emissions assuming constant or even decreasing CH 4 emissions intensities (emissions per unit product, where product is kg of protein in this case) (Chang et al., 2021). Chang et al. projected livestock methane emissions out to 2050 using different pathways of assumed emission intensity changes. These authors used two pathways with contrasting assumptions about production efficiency changes: constant emission intensity and improving efficiency (i.e., decreasing emission intensity). The “constant intensity” pathway assumed that no changes in methane emission intensities would take place in the future. The “improving efficiency” pathway was based on decreasing trends in emission intensity during the past two decades due to increasing production efficiency. Based on this finding, they constructed a “improving efficiency” pathway, assuming continuing decreases in emission intensity. Under this pathway, emissions intensities in countries showing decreasing emission intensity during the past two decades followed this decreasing trend into the future, while a constant emission intensity was applied for countries that experienced no change or an increasing emission intensity in the past two decades. Thus, other studies in the field have found it reasonable to assume constant emissions intensity of livestock products into the future. The assumption that increasing production efficiency will lead to constant or decreasing emissions intensities is not necessarily the same as the assumption that increasing production efficiency will lead

to constant emissions factors , because increasing production could still lead to increasing total (e.g., not on a per-product basis) emissions. However, enteric CH 4 emissions factors for California dairy cattle given by CARB remained constant from 2012 to 2020. Over this time, California milk production was as follows: 23,457 lbs. per head in 2012; 23,178 lbs. per head in 2013; 23,786 lbs. per head in 2014; 23,028 in 2015; 22,968 in 2016; 22,755 in 2017; 23,301 in 2018; 23,533 in 2019; and 23,990 in 2020 (USDA National Agricultural Statistics Service, 2022). Annual change in milk production, averaged over these 8 years, is 0.26%. Thus, if milk production was approximately constant, and milk emissions intensity was approximately constant or decreasing, then enteric CH 4 per cow (e.g., enteric CH 4 emissions factor) could remain approximately constant. Because CH 4 emissions factors are estimated based on dietary and production parameters, if regionally typical diets and production remain approximately the same over time, emissions factors will remain the same from year to year. CARB likely has assumed that the diets of California dairy cattle have remained approximately constant, given that the emissions factors they have calculated remain constant from 2012 to 2020. Thus, several lines of evidence underscore that it is a reasonable assumption that enteric fermentation factors will remain approximately constant to 2029 in the BAU scenario. However, this trend does not necessarily apply to other states and production situations, and enteric fermentation emissions factors may be more variable than assumed in our study. In the BAU scenario, this assumption led to approximately constant annual CH 4 emissions, and thus declining GWP ∗ emissions over time. Had enteric CH 4 emissions factors continued to rise over time, the dynamics of the scenario would be similar to the historic (1950–2017) scenario, in which enteric CH 4 emissions factors and annual CH 4 emissions did increase over time. The purpose of our “BAU” scenario was to investigate GWP ∗ dynamics relative to GWP dynamics given approximately constant annual CH 4 emissions. The “BAU” scenario utilized projected dairy cattle population data to estimate future populations under typical policy and macroeconomic conditions and projected a 0.32% decrease in population from 2018 to 2029. This small decrease in population over time, along with the constant enteric fermentation and manure management CH 4 emissions factors used, gives approximately constant annual CH 4 emissions and provides a scenario to investigate the difference in dynamics between GWP- and GWP ∗ -based estimates under constant background CH 4 emissions, unlike the historical (increasing background emissions) or reductions (decreasing background emissions) scenarios. Thus, while further investigation on trends in enteric fermentation and manure management emissions factors and future dairy cattle populations is needed, the assumption of constant California CH 4 emissions factors from 2017 to 2029 is in line with CARB emissions factors and sufficient for our study’s purposes.

Frontiers in Sustainable Food Systems

17

frontiersin.org